Theres no reason the system couldn't enable 3D in appropriate situations and revert to 2D for times when its not.

The slider already sort of set the precident for 3D being an optional perk albeit quite a significant one, as opposed to a truely integral backbone feature.

It only seems natural that the next-gen Wii would incorporate 3D to stay in line with what the 3DS is doing.

I could see this be a feature-- it is probably the cheapest way of incorporating 3D on a home console.  Multiplayer games could easily turn off the 3D effects-- as someone already mentioned 3D on the 3DS is an optional effect.

I hope the Wii 2 has more than just head-tracking though.  Kinect is probably going to incorporate this effect long before the Wii 2 will.